Smoke evacuation nozzles that have been proposed for use in aircraft are special nozzles that are normally closed; they would be opened in the event of fire, and used to remove the smoke from the passenger cabin. Studies have shown that such nozzles would be most effective if placed on or near the top of the fuselage along its centerline. As is true for most components of an airplane, the design of such a nozzle is a trade-off; in this case the trade-off is between flow capability on the one hand and weight on the other. A large nozzle will have a large flow capability, but will be heavy; and the large opening in the fuselage to accommodate it will require a considerable amount of reinforcing, which will cause its own weight penalty.
On the other hand, a small nozzle will not have the above weight penalty, but will not have the flow capability.
It is well known that a sonic orifice or nozzle flows the most fluid per unit area, and it is intuitive that such is desirable for a smoke evacuation nozzle given the above trade-off. However, the prior art has failed to produce such a nozzle that would remain effective at lower altitudes.
For example, the patent to Bruensicke, 4,552,325, shows a smoke evacuation nozzle that has a converging entrance section leading to the exit through the fuselage pressure hull. This nozzle will have sonic flow through its minimum area or throat only at higher altitudes where the ratio between the cabin pressure and the ambient pressure at a given altitude is greater than 2. At lower altitudes the flow will be subsonic, and hence the nozzle will not be as effective as a sonic nozzle.